The present invention relates to the field of projection television. More specifically, the present invention relates to the field of correcting the focus of individual color components of an image projected on a rear-projection television set.
FIG. 1 illustrates the basic design of a rear-projection television. A rear-projection television set is so named because the images are projected on to the back of the screen, while viewers watch the front of the screen.
As shown in FIG. 1, the conventional rear-projection television includes a screen (101) on which the television picture is projected. In a color projection television, the television signal (107) is divided into three signals each carrying a different color component (red, green and blue) of the image to be shown on the television.
Behind the screen (101) are three display devices (102, 103 and 104) each of which receives one of the three color component signals of the television signal (107). Each of the display devices (102 to 104) displays a single-color picture which is the color component of the picture to be shown on the screen (101) that that display device (102 to 104) received.
The display devices (102, 103 and 104) may be, for example, cathode ray tubes (CRTs). In each CRT, an electron beam is selectively scanned across a layer of phosphor to cause the phosphor to emit light to a degree proportional to the intensity of the electron beam. By varying the intensity of the electron beam as it scans the phosphor, an image is made to appear.
Each display device (102, 103 and 104) is associated with an optical projection system (105) that projects the single color-image from the respective display device (102, 103 and 104) to the rear of the screen (101). With the three different color component images projected simultaneously and superimposed on the rear of the screen (101), the viewer in front of the screen (101) sees the composite, full-color picture.
Within each of the individual CRT display devices (102, 103 and 104), an electronic or dynamic focus is system is used to focus the electron beam, i.e. control the beam width, on the phosphor layer. The dynamic focus system includes a dynamic focus signal (108) that is provided to an electromagnetic grid of each of the cathode ray tubes of the display devices (102 to 104). As shown in FIG. 1, the same dynamic focus signal (108) is provided to all three of the display devices (102 to 104).
This dynamic focus signal (108) is a waveform with two parabolic components. One component is at the horizontal scan frequency of the television set, while the other component is at the vertical scan frequency.
An inherent characteristic of projection television sets is that the output of the blue CRT is lower, i.e., less intense, than that from the red and green CRTs. This distorts the color of the resulting picture. For example, a yellowish tint may be seen in portions of an image that should be white. It is also difficult to provide a-uniform white color, where appropriate, across the entire screen.
If these problems are addressed by driving the blue CRT harder to emit more blue light, the blue spot is quickly saturated. Therefore, there have been two conventional approaches to adding more blue. The first is to make the spot size of the electron beam in the blue CRT larger than that in the red or green CRTs. An equivalent alternative is to defocus the electron beam in the blue CRT. Both of these methods in combination have also been tried.
However, the enlarged or defocused blue spot becomes distorted in ways that are detrimental to the overall picture quality. Specifically, the overly large spot becomes two big and assumes an incorrect shape, particularly around the periphery of the phosphor screen. The result is poor uniformity of the color white across the screen and blue, hazy edges around objects.
Consequently, there is a need in the art for a method and apparatus which overcomes these problems. Specifically, there is a need for a dynamic focusing system that allows the additional strength in the blue image component to be achieved without causing the flare, lack of color uniformity and hazy edges that are characteristic of pictures produced by conventional projection systems.
It is an object of the present invention to meet the above-described needs and others. Specifically, it is an object of the present invention to provide an apparatus and method that allows the additional strength needed in the blue image component to be achieved without causing the flare, lack of color uniformity and hazy edges that are characteristic of pictures produced by conventional systems.
Additional objects, advantages and novel features of the invention will be set forth in the description which follows or may be learned by those skilled in the art through reading these materials or practicing the invention. The objects and advantages of the invention may be achieved through the means recited in the attached claims.
To achieve these stated and other objects, the present invention may be embodied and described as a projection television system with dynamic focusing of the electron beams in CRT display devices respectively producing three individual color components of a picture being displayed. The system preferably includes three display devices each of which displays a different color component of the picture being displayed; projection optical systems for projecting images displayed on the display devices to a display surface; a first dynamic, focus signal applied to a first of the three display devices; and a second dynamic focus signal, different from the first dynamic focus signal, applied to a second and a third of the three display devices. The first display device displays the blue color component of the picture being displayed. By providing a different dynamic focusing signal to the display device displaying the blue color component, the blue image can be separately dynamically focused to overcome the problems of the prior art as described above. Under the principles of the present invention, the first dynamic focus signal differs from the second dynamic focus signal by being of a lesser amplification level.
The television system of the present invention preferably also includes a voltage divider circuit for generating from a dynamic focus waveform, first and second output signals of different amplification levels. The first and second output signals are processed to produce the first and second dynamic focus signals. The first and second output signals are added to a DC voltage signal to generate the first and second dynamic focus signals.
Preferably, the second dynamic focus signal may have a different DC voltage level when applied to the second display device than when applied to the third display device. To this end, the system of the present invention preferably also includes a focus block circuit for adding a dynamic focus waveform at two different amplification levels to a DC voltage to generate the first and second dynamic focus signals. The focus block circuit individually controls a DC voltage level of three output signals such that the second dynamic focus signal may be provided at two different DC voltage levels for separate application to the second and third display devices.
The present invention-also encompasses a method of dynamically focusing the electron beams in three separate CRT display devices on which three individual color components of a picture being displayed on a projection television are generated before being projected so as to be superimposed on a display surface. The method may be described as applying a first dynamic focus signal to a first of the three display devices which displays a first color component of the picture, while applying a second dynamic focus signal, different from the first dynamic focus signal, to a second and a third of the three display devices displaying, respectively, second and third color components of the picture. The first display device displays a blue color component of the picture being displayed.
The present invention may also be described as a projection television system with dynamic focusing of three individual color components of a picture being displayed including three display devices each of which displays a different color component of the picture being displayed; and projection optical means for projecting images displayed on the display means to a display surface. A first dynamic focus signal, at a first amplification level and a first DC voltage level, is applied to a first of the three display means. A second dynamic focus signal, different from the first dynamic focus signal, at a second amplification level and a second DC voltage-level, is applied to a second of the three display means. And, a third dynamic focus signal, at the second amplification level and a third DC voltage level, is applied to a third of the three display means. The first display device is the one that displays the blue color component of the picture being displayed.
As before, the first dynamic focus signal differs from the second and third dynamic focus signals by being of a lesser amplification level. The system may include a voltage divider for generating from a dynamic focus waveform, first and second output signals of different amplification levels. The first and second output signals are then processed to produce the first, second and third dynamic focus signals. The first and second output signals are added to a DC voltage signal to generate the first, second and third dynamic focus signals.
Finally, the system of the present invention may include a circuit for adding a dynamic focus waveform at two different amplification levels to a DC voltage to generate the first, second and third dynamic focus signals. This circuit individually controls a DC voltage level of the first, second and third dynamic focus signals.